Fry bacon in a skillet until crisp. Drain on paper towels and crumble. Add yellow onion to bacon drippings in skillet and cook, stirring, until soft and lightly golden brown, 3 to 4 minutes. Remove from heat and pour into bowl. Add bacon, scallions, parsley and cayenne pepper. Stir to mix. Add oysters and toss gently to mix.

Brush one pie shell with butter, then pour in oyster mixture and spread evenly with a spatula. Roll out remaining pie shell and place on top of oyster filling. Prick with a fork. Bake until crust is lightly brown – 30 to 40 minutes. Remove and cool slightly before slicing to serve.